The separation of small fluid volumes from a fluid flow poses relevant problems in many areas of chemical, biological and pharmaceutical analytics. It is frequently desirable to feed macromolecules, which have already been high-dissolution separated by a chromatographic process and are available as a fluid sample, directly to an analysis system, such as a gel electrophoresis. Devices have been described which allow such fluid dosing. According to Nilsson et al. (Journal of Biochemical and Biophysical Methods 27, 181-190 (1993)) such a device comprises a water-filled pumping chamber with attached piezoceramics. Upstream of the pumping chamber discharge a brass block is arranged through which a steel cannula is guided. This steel cannula carrying the sample flow possesses two openings arranged at right angles to the flow axis, one of which serves as connection for the pumping chamber and the other one for ejection of the microdroplets to be selected. To avoid mixing effects the fluid flow and the pumping chamber fluid are separated by a Teflon membrane of small thickness via which the pressure wave required for droplet formation can propagate. Coupling of the sample chamber to the pumping chamber is effected here by means of four bolts. The volume of the sample chamber proper is 5 .mu.l. The overall volume of the device is indicated as 34 .mu.l. This device is disadvantageous because of the complicated manufacturing technology and the relatively large volume.
Wallman et al. (Proc. Transducer 95, 303-304) describe a device in which the sample chamber is of sandwich configuration. Here two silicon wafers, into which 2 mm.times.15 mm.times.350 .mu.l large cavities have been etched, are interconnected thus building a flow duct. The upper wafer presents a hole at each end of the sample chamber, which allows inflow and outflow of the sample fluid. Between the holes a piezoceramics is arranged which allows sample ejection through an oppositely arranged 60 .mu.m diameter hole in the lower wafer. The dead volume of this device amounts to approximately 1 .mu.l.
The microfluid manipulator described in EP 0 672 834 is a device with two functional units, a microdroplet emitter and a microfluid diode. Dosing fluid droplets are injected by means of a microdroplet emitter into a droplet chamber where they wet the input surface of the microfluid diode which allows feeding the fluid medium into a target fluid. The overall volume of the microdroplet emitter and the supply duct amounts to approximately 1 .mu.l. Another disadvantage presented by this device is that thorough mixing of the fluid to be dosed in may occur in the microdroplet emitter.